This invention relates to a synchronous digital multiplexing transmission device for dealing with an input transmission signal based on a synchronous digital hierarchy (SDH) and, more particularly, to a transmission signal processing circuit for use in the synchronous digital multiplexing transmission device in processing the input transmission signal into an output transmission signal.
A transmission signal processing circuit of the type described is supplied from a transmission line with an input transmission signal which comprises an input frame (namely, a transmission line frame). The input frame (or the transmission line frame) comprises an input pointer and a datum which is typically a tributary unit (TU). The input pointer designates a location of a leading edge of the TU in the input frame. The transmission signal processing circuit processes the input transmission signal into an output transmission signal comprising an output frame (namely, a device frame). The output frame (or the device frame) comprises an output pointer and the TU. The output pointer designates another location of the leading edge of the TU in the output frame. Thus, the transmission signal processing circuit transfers the TU from the input frame onto the output frame. Inasmuch as the transmission signal processing circuit carries out renewal of a pointer from the input pointer to the output pointer, the transmission signal processing circuit is called a TU pointer processing circuit as known in the art.
In a multiplexing structure according to a synchronous digital hierarchy (SDH) based on ITU-T (Telecommunication Standardization Sector of International Telecommunication Union: previously CCITT) Recommendation G709, the TU is defined as a frame used in multiplexing digital signals. On multiplexing the digital signals having a bit rate of 1.544 Mbits/s in common, use is made of a TU-11 as the TU. When the digital signals have another bit rate of 2.048 Mbits/s in common, use is made of a TU-12 as the TU. For the digital signals having still another bit rate of 6.312 Mbits/s in common, use is made of a TU-2 as the TU. On multiplexing the digital signals which have either a higher bit rate of 34.368 Mbits/s or a still higher bit rate of 44.736 Mbits/s in common, a TU-3 is used as the TU. Thus, the TU's of four sorts or types (namely, the TU-11, the TU-12, the TU-2, and the TU-3) are present in correspondence to signal rates (or the bit rates) of the digital signals.
As will later be described in detail, a conventional transmission signal processing circuit includes a write controller which controls writing of the tributary unit of the input frame in a memory in response to a location signal representative of a location of the TU in the input frame by supplying a write address signal to the memory. A read controller controls reading of the tributary unit out of the memory in response to a stuff request signal by supplying a read address signal to the memory. A stuff execution judgement circuit is given an input stuff threshold value and Judges whether or not a stuff operation should be executed by comparing a difference between the write address signal and the read address signal with the input stuff threshold value. The stuff execution judgement circuit produces the stuff request signal when the stuff execution judgement circuit judges that the stuff operation should be executed.
In the conventional transmission signal processing circuit, the stuff execution judgement circuit is given the input stuff threshold value which is constant or fixed no matter whether the TU of the input frame has any signal rate. In other words, the input stuff threshold value is constant or fixed for the TU's of four sorts (namely, the TU-11, the TU-12, the TU-2, and the TU-3). This results in an increase of a delay required from a write timing of the TU in the memory to a read timing of the TU under consideration out of the memory when the TU under consideration has a particular signal rate selected from the bit rates of the TU's of four sorts. This is because optimum stuff threshold values are different from one another for the signal rates of the TU's of four sorts.